NBC's "Saturday Night Live" does what no other television program is able to do: it rides on the cutting edge of comedy, setting trends rather than following them. But more than that the mostly young writers and performers on the show have helped give personality and a sense of dynamics to what seems a stubbornly interim period of American history.

"Well here it is the late 1970s going on 1985," declares Dan Aykroyd early in tonight's special, "The Things We Did Last Summer," and soon he and the fat and indefatigable John Beushi have stormed the stage as The Blues Brothers with an act that is homage, satire and dead-earnest benign effrontery. These guys are merrily living out every white boy's dream of making black music.

It is sensational, funny and it defies strict analysis-clearly the highlight of "Summer," which airs at 11:30 on Channel 4 and was produced under the auspices of the "Saturday Night" crowd. Gary Weis, who has contributed many outstanding short films to the series, produced and directed and the sketches, which features the show's repertory players were written by staff scribe Don Novello.

The opener stars Gilda Radner was herself, conducting $1 tours of her apartment in Manhattan. "Let's go, let's go, let's go, let's move along," she tells the seven tourists permitted to view her duty elothers hamper, buy Gilda Radner T-shirts, and have the bathroom door slammed in their faces by their gracious hostess.

This works very well, as does Billy Murray's visit to Aberdeen, Wash., to be a baseball player-a comic docudrama in the sense that he really does play ball with a fly-by-night team and has always harbored dreams of diamond glory. He ends the sketch with a parody of Lou Gehrig's farewell speech, announcing his return to a life of comedy.

Garrett Morris gets a funny summer job, Jane Curtin smothers Memphis as a conscience-striken, hyperbolic, Geraldonian reporter ("behind me, a crowd of at least 8 million people . . . and now, black to me at another location"), and Larraine Newman suffers through a Tahitian holiday that turns into a combination horror movie and psychoanalyst's delight but which is also tedious and obscure as opposed to amusing.

Aykroyd and Belushi, however, steal the show from both ends, with excerpts from their Universal Amphitheater appearance brigtening the open and close of the program (they were the opening act for Steve Martin). They are dressed like twin Mafia funerals and both appearlargely serious about the body of music they make such a wild and ferociously sincere stab at duplicating.

All Belushi has to do is shock the stage with his pudgy agility and launch into a shamelessly inappropriate otburst of cartwheels to establish The Blues Brothers as entertaining beyond all reason and logic.

Though hesitant and littered with dead spots, "Summer" may be a more satifying 90 minutes than any of the first three "Saturday Night" shows of the new season have been. The show is so successfully now and so many of it founding zanies have become cult heros and stars that the program itself has settled into a dull dank complacency.

Proucer Lorne Michaels has domain over the fourth as well as the first three Saturdays of the month now ("Weekend" having vacated for eventual prime time), and it may be, on the basis of the evidence of "Summer," that those weeks will offer more adventurous and less predicable antics than "Saturday Night" itself. 'Bad Boys'

There are bad girls among the "Bad Boys," but toughest and most potent section of this exceptional two-hour public TV documentary is its last third, shot at the Brookwood Center, a maximum security prison for boys under 6 in upstate New York.

When you first see these convicted thieves and murderers, you're flabbergasted; some of them are mere tots. They should be on "The Electric Company," not in prison. But as they talk and show their "Born to Raise Hell" tattoos and say things like, "I'd rather have been dead-too much disappointment in life" the social and human tragedy of this situation becomes deeply alarming.

"Bad Boys," Sunday at 9 p.m. on Channel 26, was produced by Alan and Susan Raymond, who used the same "video verite" techniques on the award-winning "Police Tapes" seen earlier this year on both public and commerical TV. With portable black-and-white equipment, they get unnervingly intimate with some of the grimmest realities ever shown on television.

The first third of the report was shot at Bryant High School in New York City, a supposedly typical school where a sizable faction of students never goes to classes but hangs around near the schoolgrounds, denouncing such subjects as American history as "boring, very boring."

Then the Raymonds took their mike and camera to the Spofford Juvenile Center for 10-16-year-olds waiting to be tried or sentenced for serious crimes. "I'm here for possession of a deadly weapon," a girl says matter-of-factly, and a counselor says of the inmates, "They have no values, they're not afraid of anything, they don't care about anything."

"A Clockwork Orange" has come wall says, "Please do not give Thoratrue. A sign on the center's infirmary zine when child is withdrawing."

Some of the kids say they have been in and out of institutions since the age of 6. A few are so virulent in their belligerence that they seem clearly beyond all reach.

Most of "Bad Boys" was shot a year ago. Alan Raymond says it took 10 months to edit the tape down. Too much post-production has made the first third of the program seem fragmented and confusing, and Reymond seems never to have been satisfied with his shots, which led to a lot of nervous, needless zooming and reframing.

But at its best-near the end, when sad and crushed kids sit around talking about wasted lives-"Bad Boys" does more than propel one into dismay. It starts you thinking about the society that bred this kind of criminal and about a pop culture that persisently romanticizes antisocial behaviour, glorifies inarticulate punkdom, and seems to view Sid Vicious as a joke.

The self-exiles of Bryant Hight act delighted that their teachers refer to them as "animals and respond with an addition to street vernacular; they use "zoo" as a verb. "You zoo any time you want to," one of them says. Ironically, exposure of this on national television will probably bring it into quick popularity as a new piece of American slang. To zoo, or not to zoo. That is only part of the question.

"Bad Boys" is strikingly devoid of answers. 'F.Y.I.'

WETA makes one of its relatively infrequent contributions to the additional PBS pipeline Sunday night with "F.Y.I.," a new monthly series of programs about, according to host and executive producer Tony Bante's introduction, "problems unique to the American experience . . .and how our government responds to them."

Of course, he should have said, "if our government responds to them," but his first one-hour report, at 8 p.m. Sunday night on Channel 26, does a pretty job of wrapping up and illustrating the way inflation and other economic ills have contributed to demoralized American attitudes about their leaders and their future.

"It's not going to get any better," mutters as financially imperiled Peoria father. "It's going to get a lot worse." The program includes reports from the Midwest, Washington and Alameda County, Calif., where the allegedly liberating proposition 13 had the effect of closing down county libraries starving for funds.

It's too bad cliches like "a whopping $7 billion" and "in the final analysis" couldn't hace been eliminated from the script, but "F.Y.I.," despite some arch of formalities, brings certain aspects of its subject into crisp new focus.